1. Field of the invention
The present invention concerns cables comprising multiple pairs of individually insulated electrical conductors for transmitting high-frequency signals with low crosstalk between the pairs, for example computer cables. It concerns in particular a multiple pair cable with individually shielded pairs that is easy to connect.
2. Description of the Prior Art
Crosstalk is a key parameter in this type of cable and crosstalk can be considerably reduced by individually shielding the pairs of the cable.
The most common solution to the problem of shielding each pair is to wrap a metal or metallized tape helically around each pair before assembling the individually shielded pairs into a common protective sheath. Tape wrapping each pair is a lengthy operation, and is necessarily carried out as an additional stage on each pair already made up in order to obtain satisfactory high-frequency transmission characteristics. Tape wrapping during the construction of the pairs is not suitable since the pitch of the tape wrap is then the same as that of the conductors in each pair concerned and the regularity of the pitch required for compatibility with transmission at high bit rates cannot be guaranteed.
To connect the cable the individual shields of the pairs must be removed to obtain access to the conductors, which makes connection on site a lengthy and difficult operation.
Document GB-A-1 546 609 describes a computer cable with a plurality of individually shielded pairs. This cable is a flat cable with the pairs side-by-side. The pairs are shielded by two tapes which cover all of the pairs extending between them and are joined together on each side of each pair. Each shielding tape comprises a strip of metal, for example aluminum, coated on at least one side with a thermoplastics material and preferably on the other side with a synthetic polyester resin. The sides covered with the thermoplastics material are placed face-to-face in order to bond them by application of heat and thereby connect the two shielding tapes.
A tearing line is provided along each area of joining of the two tapes between the pairs, either on both tapes or preferably on one tape only. Such tearing lines facilitate access to the conductors of the pairs for connecting the cable.
A drawback of this cable is the result of its flat structure, which makes the cable relatively wide and flexible in one direction only so that it tends to twist during installation. This is a problem in particular in the case of a flat cable comprising four pairs, like the type of cable most frequently used in computer networks, as the cable is then very wide. It is necessary to untwist it during installation and this makes it very vulnerable to traction.
Document U.S. Pat. No. 3,819,443 describes a shielding member comprising laminated strips of metal and plastics material that are cut, bent and assembled together to define radial branches on said member. It also describes a cable including a set of conductors arranged in pairs, said shielding member and an insulative outer sheath around the set of conductors. In this cable the shielding member with the radial branches compartmentalizes the interior of the cable. The various pairs of the cable are therefore separated from each other, but each is only partially shielded, which is not so effective as shielding around each pair and is not always satisfactory.
An object of the present invention is to provide a multiple pair cable with individually shielded pairs that is easy to connect and has a circular cross-section that does not have the drawbacks of the previously described flat cable with individually shielded pairs.